Radio signals suffer from high variability in signal quality due to effects such as propagation loss, fading and interference. Although radio link quality is robust in general for most types of applications ranging from delay sensitive voice to delay tolerant data, it is not uncommon for the radio link to experience outage, manifested by dropped calls or intolerable transmission error rates. Methods used to decrease outage probability and improve reliability with a single radio link are well known. These include forward error correction and HARQ (hybrid automatic repeat request).
It is common to find multiple RAT (radio access technology) transceivers in a wireless device; it is also common to find that at any given location, it is possible to access multiple RAT networks (due to overlapping coverage areas). For example, currently most wireless handset devices support both a GSM/GPRS (Global System for Mobile Communications/General packet radio service) and a UMTS (Universal Mobile Telecommunications System) interface. Often these devices attempt to maximize coverage by accessing the best available RAT. The carrier frequency used for GSM/GPRS operation is in the 900 MHz range while the carrier frequency used for UMTS operation is in the 1.9 GHz range. The two systems are thus substantially separated in carrier frequency and are unlikely to simultaneously suffer deep fades across the respective frequency bands of operation. In some cases, the wireless link (transmit, receive) endpoints (wireless device, base station) are collocated since the services are provided by the same operator, e.g., GSM/GPRS, UMTS. At any given location and time instant, it is possible that coverage is available for multiple RATs.